samples: drivers: nucleo_h7a3zi_q: Configuring ADC sample application

I tested ADC sample on the board mentioned above
0v = 0
3.3v = 4095

Signed-off-by: Manojkumar Subramaniam <[email protected]>

Author: manoj153

samples/drivers/adc/boards/nucleo_h7a3zi_q.overlay

@@ -0,0 +1,9 @@
+/* Copyright (c) 2021 STMicroelectronics
+   SPDX-License-Identifier: Apache-2.0 */
+
+ / {
+	zephyr,user {
+		/* adjust channel number according to pinmux in board.dts */
+		io-channels = <&adc1 15>;
+	};
+};